1.1 Introduction to Cell Biology and Cell Communication

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Figure 1.3: Adenylate-cyclase signal transduction pathway.

The G-protein is phosphorylated and in turn phosphorylates the enzyme adenylate cy-

clase. The thus activated adenylate cyclase takes adenylate monophosphate (AMP) and

cyclizes it to cAMP; cAMP is an important signal in the cell that can bind and activate

many kinases. Kinases are enzymes that can phosphorylate other enzymes and thus ac-

tivate or inactivate enzymes, thereby allowing or barring specific reactions. Each of the

enzymes in the pathway can be regulated either by binding a different compound into

the active site of the enzyme or by binding to a different site in the enzyme, which is

called an allosteric site. When binding to an allosteric site, the enzyme can still react

with its compound in the active site, but the binding strength and thus the reaction is

modulated (i. e., it slows down).

Another example of a common signal transduction pathway is the phospholipase C

pathway (Figure 1.4). Again, the signal binds to a transmembrane protein that is asso-

ciated with a G-protein, a GPCR. In this case, the G-protein activates the enzyme phos-

pholipase C, which generates two signals, inositol triphosphate (IP3) and diacyl glycerol.

Diacyl glycerol in turn activates kinases as we have seen before. On the other hand, IP3

activates ion channels in the ER that release calcium (Ca²⁺) ions from storage. Ca²⁺ ions

are another important internal signal that in turn can activate another set of kinases, but

it can also control muscle contractions. Most of the senses also use signal transduction

pathways like these to react to outside signals.

To understand the signaling process at the nanoscale, it is important to look at the

nanoscale itself and the structures of some of the molecules involved, such as GPCR, in

more detail. This will allow us to then use and control these molecules in nanotechnol-

ogy.